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NASA astronomers have collected radar images of a metallic, dog
bone-shaped asteroid the size of New Jersey, an apparent leftover from an
ancient, violent cosmic collision.

The asteroid, named 216 Kleopatra, is a large object in the main asteroid
belt between Mars and Jupiter; it measures about 217 kilometers (135 miles)
long and about 94 kilometers (58 miles) wide. Kleopatra was discovered in
1880, but until now, its shape was unknown.

"With its dog bone shape, Kleopatra has the most unusual shape we've seen
in the Solar System," said Dr. Steven Ostro of NASA's Jet Propulsion
Laboratory, who led a team of astronomers observing Kleopatra with the
305-meter (1,000-foot) telescope of the Arecibo Observatory in Puerto Rico.
"Kleopatra could be the remnant of an incredibly violent collision between
two asteroids that did not completely shatter and disperse all the
fragments."

The astronomers used the telescope to bounce radar signals off
Kleopatra. With sophisticated computer analysis techniques, they decoded
the echoes, transformed them into images, and assembled a computer model of
the asteroid's shape. The Arecibo telescope underwent major upgrades in
the 1990s, which dramatically improved its sensitivity and made it feasible
to image more distant objects.

In fact, these new radar images are the first ever made of a main belt
asteroid. They were obtained when Kleopatra was about 171 million
kilometers (106 million miles) from Earth. Travelling at the speed of
light, the transmitted signal took about 19 minutes to make the round trip
to Kleopatra and back.

"Getting images of Kleopatra from Arecibo was like using a Los Angeles
telescope the size of the human eye's lens to image a car in New York,"
Ostro said.

Kleopatra is one of several dozen asteroids whose coloring suggests they
contain metal. Kleopatra's strong reflection of radar signals indicates it
is mostly metal, possibly nickel-iron alloy. These objects were once
heated, melted and differentiated into a structure containing a core,
mantle and crust, much as the Earth was formed. Unlike Earth, those
asteroids cooled and solidified throughout, and many underwent massive
collisions that exposed their metallic cores. In some cases, those
collisions launched fragments that eventually collided with Earth, becoming
iron meteorites like the one that created Meteor Crater in Arizona.

"But we don't need to worry about Kleopatra - it will never hit Earth,"
Ostro said.

"The radar-based reconstruction of Kleopatra's shape shows the object's
two lobes connected by a handle, forming a shape that resembles a distorted
dumbbell, or dog bone," said Dr. R. Scott Hudson of Washington State
University, Pullman, WA. "The shape may have been produced by the
collision of two objects that had previously been thoroughly fractured and
ground into piles of loosely consolidated rubble. Or, Kleopatra may once
have been two separate lobes in orbit around each other with empty space
between them, with subsequent impacts filling in the area between the lobes
with debris."

"The radar observations indicated the surface of Kleopatra is porous
and loosely consolidated, much like surface of the Moon, although the
composition is different" said Dr. Michael Nolan of the Arecibo
Observatory. "Kleopatra's interior arrangement of solid metal
fragments and loose metallic rubble, and the geometry of fractures
within any solid components, are unknown. What is clear is that this
object's collision history is extremely unusual."

"It is amazing that nature has produced a giant metallic object with such a
peculiar shape," said Ostro. "We can think of some possible scenarios, but
at this point none are very satisfying. The object's existence is a
perplexing mystery that tells us how far we have to go to understand more
about asteroid shapes and collisions."

The team's findings will appear in the May 5 issue of the journal Science.
Ostro's team includes Hudson; Nolan and Jean-Luc Margot of the Arecibo
Observatory; Dr. Daniel Scheeres of the University of Michigan, Ann Arbor;
Dr. Donald Campbell of Cornell University, Ithaca, NY; Dr. Christopher
Magri of the University of Maine at Farmington; and Jon Giorgini and Dr.
Donald Yeomans of the Jet Propulsion Laboratory

The Arecibo Observatory is part of the National Astronomy and Ionosphere
Center, operated by Cornell University, Ithaca, N.Y., for the National
Science Foundation. The Kleopatra radar observations were supported by
NASA's Office of Space Science, Washington, DC. JPL is managed for NASA
by the California Institute of Technology in Pasadena.

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4/24/00 JP
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